This application claims priority under 35 U.S.C. xc2xa7xc2xa7 119 and/or 365 to Patent Ser. No. 0000702-1 filed in Sweden on Mar. 2, 2000, the entire content of which is incorporated by reference.
The present invention relates to a drill element for rock drilling, and to a thread joint for interconnecting the drill element to other drill elements, wherein the thread joint is protected against corrosion. The invention also pertains to a method of protecting a threaded end of a drill element from corrosion.
During percussive rock drilling, the drill elements, i.e. bits, rods, tubes, sleeves and shanks adapters, are subjected to corrosive attacks. This applies in particular to underground drilling where water is used as a flushing medium and where the environment is humid. The corrosive attacks are particularly serious in the most stressed parts, i.e., thread bottoms and thread clearances. In combination with pulsating stress, caused by shock waves and bending loads, so-called corrosion fatigue arises. This is a common cause for failure of the drill element.
Today low-alloyed, case hardened steels are normally used in the drill element. The reason for this is that abrasion and wear of the thread parts have generally limited the life of the drill element. As the drill machines and the drill elements have become more efficient, problems due to abrasion and wear have diminished, and corrosion fatigue has become a major factor in limiting the life of the drill element.
The case hardening produces compressive stresses in the surface, which gives certain beneficial effects against the mechanical part of the fatigue. The resistance to corrosion in a low-alloyed steel is however poor and for that reason corrosion fatigue still happens easily.
In U.S. Pat. No. 4,872,515 or 5,064,004 a drill element is shown wherein a threaded portion is covered with a metallic material, which is softer than the steel of the drill element. Thus, it is intended to solve the problem of pitting in the threads by covering at least the parts of the thread of the drill element that cooperate with other parts of the threaded connection.
One object of the present invention is to substantially improve the resistance against corrosion fatigue of a drill element for percussive rock drilling.
Another object of the present invention is to substantially improve the resistance against corrosion fatigue in sections of reduced cross-sections in a drill element for percussive rock drilling.
Still another object of the present invention is to substantially improve the resistance against corrosion fatigue in the roots of the thread in a threaded portion in a drill element for percussive rock drilling.
The present invention relates to a percussive drilling component which has a male screw thread, as well as to the combination of that drilling component attached to another drilling component which has a female screw thread.
The percussive drilling component is formed of a steel material and includes an integral substantially cylindrical male screw thread. The thread comprises thread crests and thread roots interconnected by thread flanks. The thread is coated with a material having a lower electrode potential than the steel material. The coating on the thread is situated at least in regions located radially inwardly of the thread flanks.
In the case of the combination wherein the above-described drilling component is attached to another drilling component having a female screw thread, the female screw thread could also be coated with the lower potential material, with the coating situated in regions located radially outwardly of the female thread flanks.
The invention also pertains to a method of protecting a threaded end of a steel percussive drilling component against corrosion, by coating the entire threaded end with a coating material having a lower electrode potential than the steel material of the drilling component. The coating will thus be disposed on impact regions of the thread where the coating will be worn off during percussive drilling.